This invention relates to a code division multiple access (CDMA) receiver for use in a CDMA system and a method of controlling the CDMA receiver.
As a cellular mobile radio communication system, a wide variety of multiple access systems have been heretofore proposed and be adopted in the world. Among others, a recent tendency has been directed to a cellular mobile radio code division multiple access (CDMA) system which has a specific spreading code assigned to each channel and which will be simply called a CDMA system. In such a CDMA system, a modulated wave of an identical carrier frequency which is spread by each specific spreading code is transmitted as a radio signal from a transmitter side to a receiver side. Responsive to the radio signal, a CDMA receiver in the receiver side carries out synchronization operation by the use of each specific spreading code to identify a desired channel. In order to distinguish the channels from one another, different spreading codes are used to identify radio channels between the base station and the mobile stations.
In addition, it is to be noted that the radio signal is received, as received CDMA signals, through a plurality of radio propagation paths, namely, a multipath in the CDMA system and, therefore, multipath fading should be removed from the radio signal in the CDMA receiver by accurately detecting a predetermined signal, such as a synchronization signal and/or a pilot signal. The received CDMA signal is a baseband signal.
Furthermore, it is to be considered in the cellular mobile radio communication system that each mobile station is moved through the radio service areas or cells from one to another with communication kept between each mobile station and a base station. In this event, the base stations must be switched from one to another without interrupting communication with each mobile station.
Taking the above into account, a CDMA receiver which is used in a mobile station has a rake receiver supplied with received CDMA signals through the multipath and a searcher for searching such multipath signals to establish chip synchronization. The rake receiver and the searcher constitute a baseband processing portion. In other words, the searcher is used to detect optimum reception timing from the CDMA received signals and to inform the rake receiver of the optimum reception timing. That is, the searcher calculates a delay profile in the received CDMA signals to produce path information corresponding to a main propagation path. This applies to the CDMA receiver which is used in each base station.
Practically, both the rake receiver and the searcher are given received CDMA signals which are subjected to high frequency amplification and frequency conversion by a high frequency amplifier and an intermediate amplifier, respectively. In this event, the rake receiver is operable in response to the received CDMA signals received through the multipath and comprises a plurality of finger receivers for demodulating the received CDMA signal by the use of a spreading code into a demodulated signal. To this end, calculations are made about correlations between a spreading code of a desired channel and each received multipath signal to capture codes at each reception timing through each path. Thereafter, maximal-ratio combining or rake combining is carried out to augment received signal strength. Such maximal-ratio combining or rake combining is effective to alleviate an influence of the multipath fading and to improve a signal-to-noise (S/N) ratio.
On the other hand, the searcher has a plurality of correlators which are operable in response to the received CDMA signals and delayed spreading codes to calculate correlation values between the received CDMA signals and the delayed spreading codes and a plurality of adders for adding the respective correlation values to produce summed up correlation values. Moreover, the summed up correlation values are delivered to an effective path judgment circuit to determine an effective path and to supply the finger circuits of the rake receiver with reception timing signals representative of the effective path or optimum reception timing.
Herein, it is to be noted that the correlators of the searcher are in one-to-one correspondence to the adders.
In a wide-band CDMA (W-CDMA) communications apparatus or receiver, a baseband signal has a signal rate which is higher than that of a narrow-band CDMA communications apparatus or receiver. In addition, the W-CDMA receiver has a baseband processing portion which consumes an increased power. As a result, there are circumstances which cannot despise a method of decreasing a consumed power. Furthermore, in a CDMA communications apparatus operable in a battery such as a portable terminal, it is properly desirable to decrease a consumed power in the apparatus without distinction of the wide-band and the narrow-band.
In order to meet such a demand, Japanese Unexamined Patent Publication of Tokkai No. Hei 11-186,987 or JP-A 11-186987 discloses “PHASE TRACKING DEVICE FOR CDMA RECEIVER” to relieve the processing load of an integration means by varying an integration time depending on the reception environment, in phase tracking to maintain synchronization of a CDMA (code division multiple access) receiver. According to JP-A 11-186987, a despreading means despreads a pilot signal in a received signal to produce a despread pilot signal. A strength measurement means measures a received pilot signal strength from the despread pilot signal. Based on the received pilot signal strength, an integration control means determines an integration time interval in an integration means. The integration means sums, for the integration time interval, the strength difference detected by a strength difference detection means. Inasmuch as the integration time interval in the integration means is varied in response to the reception environment, the processing load of the integration means is relieved when the reception environment. It is therefore possible to realize a superior phase tracking device with less power consumption for a CDMA receiver.
In addition, various other prior arts related to the present invention are already known. By way of example, Japanese Unexamined Patent Publication of Tokkai No. Hei 11-177,524 or JP-A 11-177524 discloses “MOBILE TERMINAL EQUIPMENT” to extend the service time by reducing power of the mobile communication terminal equipment in an intermittent reception. According to JP-A 11-177524, the mobile communication terminal equipment adopting the CDMA system is provided with a pilot PN synchronization detection section including a SAW matched filter that carries out partial correlation arithmetic with a pilot PN code at an intermediate frequency band. The pilot PN synchronization detection section produces an initializing pulse in matching with production of a correlation peak for a pause period at intermittent reception. A PN timing generating circuit is initialized by the initializing pulse to maintain pilot PN synchronization. As a result, power supply of a baseband front-end section is interrupted, the operation of a rake engine section is stopped to reduce the current consumption for a pause period.
Japanese Unexamined Patent Publication of Tokkai No. Hei 10-200,507 or JP-A 10-200507 discloses “RECEIVER, RECEIVING METHOD AND TERMINAL IN RADIO SYSTEM” to shorten the operation time of a controller at the time of intermittent reception and to reduce power consumption by setting control information of the next receiving state during the operation of a controlling part before, reaching a sleep state and performing intermittent reception so that the operation of the controlling part may be stopped until data processing is needed. According to JP-A 10-200507, a searcher is activated at the time of intermittent reception, and after searching processing of the searcher is finished, an operation of a controller is started. After the searcher captures each path, a phase that is captured by the searcher is set to a plurality of fingers, and the operations of the fingers are started. The controller does not operate at the time of search in this way, and after the search has been finished, the operation time of the controller is started. Then, the operation time of the controller spans from when the search of the searcher is finished to when a demodulation operation at the fingers is completed, the operation time is shortened and power consumption is reduced.
Japanese Unexamined Patent Publication of Tokkai No. Hei 7-95,126 or JP-A 7-095126 discloses “RECEIVER” to reduce power consumption by stopping the operation of a TTL circuit for undesired control. According to JP-A 7-095126, l upon the receipt of a decision bit whose level is logical “1”, a searcher stops the operation of first through third TTL (Time Tracking Loop) circuits provided respectively to first through third despreading devices except for the first TTL circuit. And the phase of a PN series generated by a PN generating circuit controlled by the TTL circuit whose operation is stopped is controlled so as to be coincident with a position shifted in the unit of delay time between paths constituting a multi-path based on the phase of the PN series controlled by the first TTL circuit in operation.
However, in the above-mentioned Publications, power reduction of the searcher is not considered. Inasmuch as the searcher detects the paths at a high speed, the searcher has a large scale in circuitry and has a large power consumption. As a result, it is desired to reduce power consumption in the searcher.